Reduced height electrical insulating bushing with sight glass



A g- 0, 1963 E. F. CRONlN ETAL 8 REDUCED HEIGHT ELECTRICAL INSULATING BUSHING WITH SIGHT GLASS Filed Oct. 9, 1961 26 .1 29 Fzg Z6 PIP/0R MP7 United States Patent York Filed Oct. 9, 1961, Ser. No. 143,767 i 6 Claims. (Cl. 174 -11) This invention relates to electrical insulating bushings, and more in particular to liquid-insulated bushings having means for visibly indicating'the level of the insulating liquid.

In many types of electrical bushings, the central conductor is normally surrounded throughout part of its length with layers of liquid-permeable material, such as Kraft paper. To'obtain proper insulating strength, liquidinsulation, such as transformer oil, must be kept above the top of the oil-permeable material to prevent electric arcing and breakdown within the bushing. It is obviously desirable that means he provided for indicating from the outside of the bushing, the level of the insulating liquid inside the bushing; in high voltage applications, this has not been accomplished by making thebushing entirely from transparent material because transparent materials do not have all of the needed electrical or mechanical properties.

Numerous expedients have been employed in the past for indicating the level of the liquid inside a bushing. For example, various types of mechanical liquid level gauges usually employing some sort of float mechanism have been used. The mechanical arrangements are undesirable because a mal-functioning may result in the mechanism failing to indicate that the liquid has reached an unsafe level. I

One type of arrangement that obviates the disadvantages of the above-mentioned mechanical arrangements is the use of a Window made from a transparent material such as glass above the insulating shell of the bushing to permit actual observation of the level of the liquid. A separate transparent shell must be provided because the ceramic insulating materials, such as porcelain, commonly used for bushing insulators are opaque. The prior art arrangements employing transparent shells have several disadvantages. One disadvantage is that the transparent shell is not useable as a part of the insulating shell surrounding the central conductor. Therefore, the height of the bushing is increased by the height of the transparent shell without a corresponding gain in effective insulation. This is undesirable because of the added cost of materials; furthermore, any increase in height of the apparatus employing the bushings causes problems with road clearances, etc. Another disadvantage of prior art arrangements in which the transparent shell is at the top of the bushing is that the isulating liquid must be maintained at an unnecessarily high level to be visible through the transparent shell; the reason is that the liquid-permeable material surrounding the central conductor ordinarily does not extend the full height of the bush hldlfidh Patented Aug. 20, 1963 2 ment being such that extra liquid in addition to that needed to insulate the central conductor is not required for proper operation of the sight glasses.

A further object is to provide -liquid-insulated high voltage bushings with transparent sight glasses in which the sight glasses function as eifective insulating shells around the central conductor.

A further object is to provide liquid-insulated high voltage bushings with transparent sight glasses that are protected from contamination by moisture.

Another object is to provide liquid-insulated bushings with transparent shells Whose exterior surfaces are shielded from moisture contamination so that the exterior surfaces are an effective part of the creep failure-preventing surfaces of the bushings.

Other objects and advantages of the invention will become apparent from the drawing, specification, and claims, and the scope of the invention will be pointed out in the claims.

Our invention is based on the discovery that the exterior surface of a transparent shell of the type employed as a sight glass on liquid-insulated bushings can be included in the effective creep failure-preventing surface of the insulating shells surrounding the central conductor, when the transparent shell is placed intermediate other insulating shells, and when the insulating shell above the transparent shell includes the means that prevents moisture from contaminating the exterior surface of the transparent shell.

Briefly stated, according to one aspect of our invention, a high voltage bushing of the type having a central conductor that passes through a wall may employ a pair of hollow opaque insulating shells to insulate the centralconductor. A hollow, transparent shell may be placed intermediate the opaque shells. The shells define on the interior thereof a liquid chamber that contains an electrical insulating liquid for the central conductor. The arrangement is such that the level of the liquid is visible through the transparent shell. The opaque shell above the transparent shell is provided with at least one integral moisture-flow interrupting flange extending radially outwardly beyond the exterior surface of the transparent shell. This prevents moisture from contaminating the exterior surface of the transparent shell; this permits the transparent shell to function as an insulating member in determining the effective creep failure preventing insulating length of the column of shells around the central conductor. Thus, the height of the bushing need not be increased by the height of the transparent shell, and the only insulating liquid needed is that required to insulate the central conductor.

In the drawing:

FIGURE 1 is a schematic, partially cross-sectional, side elevational view of a high voltage electrical insulating bushing constructed in accord with the prior art.

FIGURE 2 is a schematic, partially cross-sectional,

V side elevational View of a high voltage electrical insulating ing, and the liquid level need not be any higher than bushing constructed in accord with the teachings of our invention. 7 I

In the drawing, FIG. 1 shows the conventional prior art arrangement of a liquid-insulated, high voltage electrical bushing 10. According to conventional practice, the bushing 10 includes attaching means such as a metal mounting flange 11 for attaching same to a wall 12 of an enclosure for electrical lapparatus, such as a power transformer. The wall 12 will be at a reference ground potential. Bolts 14 secured to the wall 12 may extend upwardly through suitably placed holes in the flange 11, and nuts 15 may be threaded on the bolts 14 for securing the bushing in place. A hollow opaque insulating shell 16 made from ceramic material such as porcelain is placed on the flange 11. A hollow shell 17 of trans- 'nearest point at ground potential.

parent material such as glass is placed on the shell 16 to provide a window or sight glass for viewing the level of liquid insulation in the bushing. On the inside of the wall 12, ahollow insulating shell 18is employed under the mounting flange 11.

The hollow shells define a chamber 20 on the interior thereof through which the bushing central conductor 25 extends. The upper end 26 and lower end 27 of the central conductor are threaded for engagement with nuts 28 and 29. A line lead 30 is held on the centnal conductor 25 by the nut 29, thus raising to line potential the central conductor and all metal parts of the bushing it touches. A lead 31 to the electrical apparatus is held on the central conductor by the nut 28. The central conductor 25 also serves the function of center clamping the shell elements together by means of nuts 32 and 33 at opposite ends thereof that are tightened on the threaded ends 26 and 27. The clamping action compresses gaskets 33, 34,and 35 between the shells surrounding the \central conductor to make the chamber 20 liquid-tight. To accommodate expansion and contraction of the central conductor and shells on temperature changes, resilient expansion means 37 of any conventional construction may be employed at one end of the bushing.

In accord with conventional practice, the central conductor 25 is wrapped over a portion of its length with liquid permeable material 40, such as kraft paper. Stress equalizers 41 may be embedded in material 40 for grading electrical stresses within the bushing. To adequately insulate the central conductor, the material 40 must be saturated with an electrical insulating liquid 42, such as transformer oil. For insulation purposes, the liquid 42 must be maintained above a predetermined level slightly above the upper end 43 of the material 41). No liquid is needed above the end 43 for insulation purposes. However, in the prior art arnangement of FIG. 1, the location of the transparent shell 17 on top of the upper end of the shell 16 requires that the liquid level be maintained within the height of the shell 17 before it can be viewed therethrough externally of the bushing. This means that more liquid must be maintained in the bushing than is needed for insulating purposes, solely to enable the shell 17 to function as a window or sight glass.

Another disadvantage of this proir art arrangement is that the height of the bushing is increased by the height of the tnansparent shell 17 because the shell 17 does not function as an effective insulating member around the central conductor 25. The reason is that the shell 17 will be contaminated by moisture (e.g. rain). Therefore the exterior surface of shell 17 cannot be included in the predetermined length of insulating material necessary to prevent wet creepage failure or flashover between the point on the bushing at line potential and the In the illustrated embodiment, the bottom of the expansion means 37 would be the point at line potential nearest to a point at ground potential (eg. the top of the mounting flange 11). This has led to the practice of not including the transparent shell 17 as a part of the column of material providing the predetermined required strike distance (i.e. straight line height) between parts at line potential and those at ground potential. In FIG. 2, the predetermined strike distance is designated X.

Turning now to FIG. 2, therein is shown an electrical insulating bushing 100 in accord with the teachings of our invention. -In FIG. 2, parts corresponding to those in FIG. 1 [have been given corresponding numbers in the 100 series. Those parts whose structure or function is altered in our arrangement are numbered differently.

It should be noted that in our arrangement the predetermined strike distance X includes the transparent shell 119, which is identical in structure to the shell 17. This reduces the height of the bushing by the height of the shell 121, and placing a second hollow insulating shell 122 on the transparent shell 119. The shells 121 and 122 may be made of an opaque ceramic material such as porcelain. The second shell 122 has thereon at least one integral moisture-flow interrupting flange 123 that extends radially, outwardly therefrom. These flanges are commonly referred to in the art as petticoats. The flange 123 has on the underside thereof a drip-causing edge 12 that prevents moisture from running down on the transparent shell 119 and thereby contaminating the exterior surface of the shell 1.19. The drip edge 124 may be of many dilferent configurations, however, the reverse contour lip illustrated in the drawing is our preferred embodiment.

The moisture-flow interrupting flange 123 which shields the transparent shell 119 like an umbrella is the structure that enables the shell 119 to be included as effective insulation in the column of insulating material through which the central conductor 125 passes. The reason is that tests have shown that the exterior surface of the shell 119 will function properly under Wet conditions (e.g. during rain) as a creepage failure-preventing surface, thus eliminating the need for several petticoat flanges. We have found that for practical purposes, the height of the column consisting of the first shell 121, the second shell 122, and the transparent shell 119 need be no more than the height X of the prior art shell 16; since the height X is the required strike distance, the tranparent shell 119 serves the same insulating functions performed by the. first and second shells. In short, the overall result is that the height of the bushing 100 is re duced by the height of the transparent shell 119.

Our arrangement has the further advantage of reducing the amount of electrical insulating liquid 142 required.

This is accomplished by determining the height of the first shell 121 so that its upper edge 139 extends slightly above the upper edge 143 of the liquid permeable material 140, which is the predetermined mini-mum liquid level necessary for insulation purposes. This locates the lower edge 145 of the transparent shell 119 just above the predetermined minimum level and ensures that visual inspection from the outside'of the bushing will indicate when the liquid has fallen to an unsafe level. This eliminates the prior art requirement that additional liquid 42 he provided to raise the liquid level above the top of the porcelain column merely for the purpose of being visible through the transparent member 17, as shown in FIG. 1.

It has thus been shown that by the practice of our invention, the height of a bushing can be reduced and the amount of liquid necessary in the bushing can be reduced by placing the transparent Window or sight glass in the insulating column intermediate two ceramic shells. This enables the moisture-flow interrupting flange on the upper shell to function as an umbrella for shielding the transparent shell from moisture contamination, and thereby enables the transparent shell to function as insulation preventing wet creepage failure, as well as strike failure.

It will be understood, of course, that while the forms of the invention herein shown and described constitute preferred embodiments of the invention, it is not intended herein to illustrate all of the equivalent'forms or ramifications thereof. For example, although the bushings are illustrated as having a vertical axis, those skilled in the art will realize that the invention is applicable to bushings having tilted axes. It will also he understood that the words used are words of description rather than of limitation, and that various changes may be made without departing from the spirit or scope of the invention herein disclosed, and it is aimed in the appended claims to cover all such changes as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A liquid-insulated electrical bushing comprising a central conductor that passes through a wall at a reference ground potential, said central conductor being at line potential during energization thereof, means for attaching said bushing to said w-all, said central conductor passing through a column of insulating material having an exterior surface of length sufficient to' provide a predetermined creepage failureapreventing distance between the nearest points on said bushing respectively at ground and line potential, said column comprising a first hollow opaque insulating shell on said attaching means, a hollow transparent shell on said first shell, and a second hollow opaque insulating shell on said transparent shell, said column defining a liquid chamber on the interior thereof, an electrical insulating liquid in said chamber, said liquid filling said column to a predetermined level to adequately insulate said central conductor, the height of said first shell being predetermined so that said predetermined liquid level occurs within said transparent shell, whereby visual inspection from the outside of the bushing will indicate whether sufficient insulating liquid occupies said chamber, said second shell having at least one integral moisture-flow preventing flange extending radially outwardly therefrom beyond the exterior surface of said transparent shell to prevent moisture from contaminating said exterior surface, whereby said transparent shell exterior surface acts as effective insulating material in the length of said column of insulating material providing said predetermined creepage failure-preventing distance between said nearest ground and line potential points.

2. A high voltage bushing comprising a central conductor that passes through a wall at a reference ground potential, said central conductor terminating on the outside of said bushing and being at line potential during energization thereof, means for attaching said bushing to said wall, said central conductor passing through a column of insulating material having 'an exterior surface of length suflicient to provide a predetermined creepage failurepreventing distance between the nearest points on said bushingrespectively at ground and line'potential, said column comprising a first hollow ceramic shell on said attaching means, a hollow transparent shell on said first shell, and a second hollow ceramic shell on said transparent shell, said column defining a liquid chamber on the interior thereof, an electrical insulating liquid in said chamber, said liquid-filling said column to a predetermined level to adequately insulate said central conductor, the height of said first shell being predetermined so that said predetermined liquid level occurs within said transparent shell, whereby visual inspection from the outside of the bushing will indicate whether sufficient insulating liquid occupies said chamber, said second shell having at least one integral flange extending radially outwardly therefrom, said' flange having a reverse contour dripcausing edge on the underside thereof, said drip-causing edge extending radially outwardly beyond the exterior surface of said transparent shell to prevent moisture from contaminating said exterior surface, whereby said transparent shell exterior surface acts aseifective insulating material in the length of said column of insulating material providing said predetermined creepage failure-preventing distance between said nearest ground and line potential points.

3. A high voltage bushing comprising a central conductor that passes generally vertically through a wall at a reference ground potential, said central conductor terminating on the outside of said bushing and being at line potential duning energization thereof, a mounting flange for attaching said bushing to said Wall, said central conductor passing through a column of insulating material having an exterior surface of length sufiicient to provide a predetermined creepage failure-preventing distance between the nearest poin-ts on said bushing respectively at ground and line potential, said column comprising a first hollow ceramic shell on said mounting flange and extending above the mounting flange, a hollow transparent shell on said first ceramic shell and extending above said first shell, and a second hollow ceramic shell on said transparent shell [and extending above said tnansparent shell, said column defining a liquid chamber on the interior thereof, an electrical insulating liquid in said chamber, said liquid filling said column to a predetermined level to adequately insulate said centnal conductor, the height of said first shell being predetermined so that said predetermined liquid level occurs within said transparent shell, whereby visual inspection from the outside of the bushing will indicate whether sufficient insulating liquid occupies said chamber, said second shell having at least one integnal [flange extending radially outwardly therefrom, said flange having a reverse contour drip-causing edge on the underside thereof, said drip-causing edge extending radially outwlardly beyond the exterior surface of said transparent shell to interrupt the flow of moistureon said exterior surface, whereby said transparent shell exterior surface acts as effective insulating material in the length of said column of insulating material providing said predetermined creep age failure-preventing distance between said nearest ground andline potential points.

4. A liquid-insulated high voltage bushing comprising a central conductor that passes through a wall at a reference ground potential, said central conductor terminating on the outside of said bushing and being at line potential during enengization thereof, said central conductor being wrapped [for a predetermined length with liquid-permeable material, means for attaching said bushing to said well, said central conductor passing through a column of insulating matenial having an exterior surface of length sufficient to provide a predetermined creepage failure-preventing distancebetween the nearest points on said bushing respectively at ground and line potential, said column comprising a first hollow ceramic shell on said attaching means, a hollow transparent sight glass on said first shell, and a second hollow ceramic shell on said sight glass, said colunm defining a liquid chamber on the interior thereof, \an electrical insulating liquid in said chamber, said liquid filling said column to a predetermined level slightly above said predetermined length of wrapped conductor to adequately insulate said central conductor by saturating said liquid permeable material, the height of said first shell being predetermined so that said predetermined liquid level occurs within said sight glass, whereby visual inspection from the outside of the bushing will indicate whether sufiicient insulating liquid occupies said chamber and said sight gllasses will function properly with only the amount of liquid required for insulating purposes, said second shell having at least one integral flange extending radially outwardly therefrom, said flange having a reverse contour drip-causing edge on the undersidev thereof, said dripcausing edge extending radially outwardly beyond the exterior surface of said sight glass to prevent moisture from contaminating said exterior surface, whereby said sight glass exterior surface acts as effective insulating material in the length of said column of insulating material providing said predetermined adequate creepage failure-preventing distance between said nearest ground and line potential points.

5. A reduced height oil-filled electrical bushing having a composite insulating shell for an axially extending central conductor, said shell extending between an end adjacent a conducting mounting flange and an end adjacent a conductig line terminal, said shell having three axially aligned circular cross section portions which progressively increase in axial length from the shortest which is adjacent the line terminal end to the longest which is adjacent the mounting flange end, said shortest and longest end portions consisting of opaque insulating material, the intermediate portion of intermediate length consisting of transparent material having a smooth outer surface {for facilitating visual inspection of oil level, the longest portion having a plurality of axially spaced integral flanges extending radially outwardly therefrom, the shortest portion having at least one integral flange extending radially outwardly therefrom, said flanges each having a reverse contour drip-causing edge on the underside thereof, said composite insulating shell having Wet and dry outside surface insulation strength which is substantially equal tothe corresponding insulation strength of a conventional shell of the same dimensions and configuration but consisting entirely of said I opaque insulating material.

6. A reduced height oil-filled electrical bushing having a composite insulating shell for an axially extending central conductor, said shell extending between an end adjacent a conducting mounting flange and an end adjacent a conducti g line terminal, said shell having three axially aligned circular cross section portions which progressively increase in axial length from the shortest which is adjacent the line terminal end to the longest which is adjacent the mounting flange end, said shortest and longest end portions consisting of opaque insulating material, the intermediate portion of intermediate length consisting of transparent material having a smooth outer surface for facilitating visual inspection of oil level, the longest portion having a plurality of axially spaced integral flanges extending radially outwardly therefrom, the shortest portion having at least two axially spaced integralflanges extending radially outwardly therefrom, said flanges each having a reverse contour drip-causing edge on the underside thereof, said composite insulating shell having wet and dry outside surface insulation strength which is substantially equal to the corresponding insulation strength of a conventional shell of the same dimensions and configuration but consisting entirely of said opaque insulating material.

References Citedin the file of this patent UNITED STATES PATENTS 2,189,610 Lin-gal et a1. Feb. 6, 1940 2,189,632 'Ford- Feb. 6, 1940 FOREIGN PATENTS 650,623 Germany Sept. 27, 1937 

5. A REDUCED HEIGHT OIL-FILLED ELECTRICAL BUSHING HAVING A COMPOSITE INSULATING SHELL FOR AN AXIALLY EXTENDING CENTRAL CONDUCTOR, SAID SHELL EXTENDING BETWEEN AN END ADJACENT A CONDUCTING MOUNTING FLANGE AND AN END ADJACENT A CONDUCTING LINE TERMINAL, SAID SHELL HAVING THREE AXIALLY ALIGNED CIRCULAR CROSS SECTION PORTIONS WHICH PROGRESSIVELY INCREASE IN AXIAL LENGTH FROM THE SHORTEST WHICH IS ADJACENT THE LINE TERMINAL END TO THE LONGEST WHICH IS ADJACENT THE MOUNTING FLANGE END, SAID SHORTEST AND LONGEST END PORTIONS CONSISTING OF OPAQUE INSULATING MATERIAL, THE INTERMEDIATE PORTION OF INTERMEDIATE LENGTH CONSISTING OF TRANSPARENT MATERIAL HAVING A SMOOTH OUTER SURFACE FOR FACILITATING VISUAL INSPECTION OF OIL LEVEL, THE LONGEST PORTION HAVING A PLURALITY OF AXIALLY SPACED INTEGRAL FLANGES EXTENDING RADIALLY OUTWARDLY THEREFROM, THE SHORTEST PORTION HAVING AT LEAST ONE INTEGRAL FLANGE EXTENDING RADIALLY OUTWARDLY THEREFROM, SAID FLANGES EACH HAVING A REVERSE CONTOUR DRIP-CAUSING EDGE ON THE UNDERSIDE THEREOF, SAID COMPOSITE INSULATING SHELL HAVING WET AND DRY OUTSIDE SURFACE INSULATING STRENGTH WHICH IS SUBSTANTIALLY EQUAL TO THE CORRESPONDING INSULATION STRENGTH OF A CONVENTIONAL SHELL OF THE SAME DIMENSIONS AND CONFIGURATION BUT CONSISTING ENTIRELY OF SAID OPAQUE INSULATING MATERIAL. 